Lactobacillus acidophilus is a Gram-positive, rod-shaped, non-spore forming, homofermentative bacterium that is a normal inhabitant of the gastrointestinal and genitourinary tracts. Since its original isolation by Moro (1900) from infant feces, the “acid loving” organism has been found in the intestinal tract of humans, breast-fed infants, and persons consuming high milk, lactose, or dextrin diets. Historically, L. acidophilus is the Lactobacillus species most often implicated as an intestinal probiotic capable of eliciting beneficial effects on the microflora of the gastrointestinal tract (Klaenhammer and Russell (2000) “Species of the Lactobacillus acidophilus complex,” Encyclopedia of Food Microbiology, Volume 2, pp. 1151-1157. Robinson et al., eds. (Academic Press, San Diego, Calif.). L. acidophilus can ferment hexoses, including lactose and more complex oligosaccharides, to produce lactic acid and lower the pH of the environment where the organism is cultured. Acidified environments (e.g., food, vagina, and regions within the gastrointestinal tract) can interfere with the growth of undesirable bacteria, pathogens, and yeasts. The organism is well known for its acid tolerance, survival in cultured dairy products, and viability during passage through the stomach and gastrointestinal tract. Lactobacilli and other commensal bacteria, some of which are considered as probiotic bacteria that “favor life,” have been studied extensively for their effects on human health, particularly in the prevention or treatment of enteric infections, diarrheal disease, prevention of cancer, and stimulation of the immune system.
The cell wall of Gram-positive bacteria consists of a peptidoglycan macromolecule, with attached accessory molecules such as teichoic acids, teichuronic acids, lipoteichoic acids, lipoglycans, polyphosphates, and carbohydrates (Hancock (1997) Biochem. Soc. Trans. 25:183-187; Salton (1994) The bacterial cell envelope-a historical perspective, p. 1-22. In J. -M. Ghuysen and R. Hakenbeck (ed.) Bacterial cell wall. Elsevier Science BV, Amsterdam, The Netherlands). Proteins associated with the cell surface of Gram-positive bacteria include hydrolases and proteases, polysaccharides, surface exclusion proteins and aggregation-promoting proteins (thought to be involved in mating), S-layer proteins (subunits of crystalline arrays covering the outer surface of many single-celled organisms), sortase (a transpeptidase responsible for cleaving surface proteins at the LPXTG-like (SEQ ID NO:308) motifs), proteins with LPXTG-like motifs, and MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) such as fibronectin-binding proteins, fibrinogen-binding proteins, and mucus-binding proteins.
Cell wall, cell surface, and secreted proteins of Gram-positive bacteria serve many diverse functions, including adhering to other cells or compounds, providing structural stability, and responding to environmental stimuli. Surface proteins of bacteria are important for survival within a host, and for cell growth and division. Furthermore, surface proteins are often recognized by a host's immune system to initiate immuno-stimulation, -modulation, or -enhancement. The isolation and characterization of these proteins will aid in developing essential probiotic products with numerous applications, including those that benefit human or animal health, and those concerned with food production and safety.